site: clearer examples for <slot let:name>

[mdempsky]

In the first example, "item" is used to describe three logically
distinct objects: (1) the name of the slot's property; (2) the JS
variable used within FancyList.svelte's {#each} block; and (3) the JS
variable used within App.svelte. To make it clearer that these are
three different objects, give them different names.

The second example can continue reusing the same name for all of the
objects to demonstrate there's no collision between them. But we can
also simplify the example to take further advantage of shorthand
rules.

[mdempsky]

For context, I found learning about this feature confusing. I'm used to "X=Y" means "set variable X to value Y". However "let:foo={bar}" actually means "declare variable 'bar' and set it to the value of property 'foo'". The examples didn't make this clear because "let:item={item}" confuses which instance of the identifier "item" does what. I had to copy the code into the REPL and play around with it to figure this out.